Introduction to air-cooled heat exchangers
Water shortage and increasing costs, together with more recent concerns about water pollution and cooling tower plumes, have greatly reduced industry's use of water cooled heat exchangers. Consequently, when further heat integration within the plant is not possible, it is now usual to reject heat directly to the atmosphere, and a large proportion of the process cooling in refineries and chemical plants takes place in Air Cooled Heat Exchangers (AC-HEs).
There is also increasing use of Air Cooled Condensers for power stations. The basic principles are the same but these are specialized items and are normally configured as an A-frame or "roof type". These condensers may be very large-the condensers for a 4000 MW power station in South Africa have over 2300 tube bundles, 288 fans each 9.1 m in diameter and a total plot area 500 m X 70 m.
AC-HEs for process plants are normally just called Aircoolers, but should not be confused with devices for cooling air (best described as Air Chillers).
The design of an AC-HE is more complex than for a Shell and Tube Heat Exchanger, as there are many more components and variables.
The structure of an AC-HE is painted or galvanized, depending on customer specification. However, the costs are roughly the same if a multiple coat paint system is specified. Often the painted units are more expensive. There seems to be a trend toward more galvanized structures because they require virtually no maintenance. Painted structures require touch-up after installation and they often rust anyway.
Air-cooled heat exchangers are used extensively throughout the oil and gas industry, from upstream production to refineries and petrochemical plants, under high pressure and high temperature conditions, as well as corrosive fluids and environments.
Applications:
Petrochemical
Refining
Refinery
chemical plants
Power generation
Natural gas processing
Natural gas compression
Oil & gas transmission
LNG liquefaction &vaporization
Engine radiator cooling
Bitumen extraction and upgrading
Industrial process applications
natural gas processing plants
liquefaction facilities
large transmission pipelines
Gas Compression
What standards air used for Air-Cooled Exchangers?
First, almost all air coolers are built to Sect. VIII of the ASME Code, since they are pressure vessels. For refinery and petrochemical services most customers include API 661 (Air-Cooled Heat Exchangers for General Refinery Service) in their specifications.
This API spec is very good since it includes all the necessary information to properly specify a cooler and provides for a high level of minimum quality in the design and fabrication of the cooler. In the back it has a very good checklist where a customer can decide exactly what type construction is needed and what options are important. These include such items as galvanizing vs. painting, types of headers, maintenance walkways and platforms, controls, and external loads on the cooler. The following details refer mostly to the API specifications.
What kinds of finned tubes are used?
The tubes can be of virtually any material available, such as carbon steel, stainless steel, Admiralty brass, or more exotic alloys. The minimum preferred outside diameter is one inch. Some manufacturers sometimes use smaller tubes, but most of the process coolers have tubes which are 1.0", 1.25", or 1.5" OD. The minimum tube wall thicknesses vary with the material. In some cases the design pressure and design temperature of the exchanger govern the minimum thickness.
The fins are almost always of aluminum material. The most common type of fin is the helically wrapped, L-footed type. These are used where the process temperatures are below about 350 °. F. The API specification calls for cast zinc bands at the ends of the tubes to prevent the fins from unwrapping. Some of the better manufacturers also use cast zinc bands at the tube supports. For higher process temperatures, most customers prefer either embedded or extruded fins. The embedded fins have the highest temperature capabilities. They are made by a process which cuts a helical groove in the OD of the tube, wraps the fin into the groove, then rolls the upset metal from the tube back against the fin to lock it into place. The tube wall must be thicker with embedded fins because of the groove.
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